The present invention relates to medical prostheses and, in particular, it concerns a prosthesis for replacing a joint between a first bone and a second bone in a human hand or foot.
The first elongated bone (metacarpal) at the base of the thumb is connected to the wrist trapezium through the carpometacarpal (CMC) joint which is sometimes referred to as the trapezio-metacarpal joint. This particular joint is of critical anatomical importance to humans, due to the opposing motion of the thumb with respect to the fingers for grasping objects and performing daily functions. Damage to the CMC joint through physical injury or disease can therefore be a severe physiological burden to inflicted humans.
Osteoarthritis, rheumatoid arthritis, and post-traumatic arthritis of the carpometacarpal (CMC) joint cause interminable pain and poor function of the thumb. Patients who have mild symptoms often respond to rest, immobilization, non-steroidal anti-inflammatory drugs, or intra-articular injections of steroids. However, patients who have more severe forms of trapezio-metacarpal (carpometacarpal) arthritis may have dorsoradial subluxation of the joint, adduction contracture of the first web space, severe loss of articular cartilage, secondary metacarpophalangeal hyperextension and interphalangeal flexion deformity accompanied with disabling pain and hand function weakness. Carpometacarpal arthritis is often an isolated entity, without significant concomitant involvement of the trapezio-scaphoid or trapezio-trapezoid joint. Moreover, retaining the trapezium in patients who have isolated trapezio-metacarpal arthritis is justified in order to preserve stability at the base of the thumb. Any carpometacarpal total joint replacement should restore the length and direction of the first metacarpal by reducing any existing subluxation of the metacarpal on the trapezium and by providing a fixed fulcrum for the metacarpal to articulate with the trapezium.
The goal of surgical intervention in patients who have CMC arthritis is to restore a strong, painless and stable thumb without deformity or decreased motion. Many reconstructive procedures have been devised including interposition arthroplasty, resection arthroplasty of the trapezium, resection interposition arthroplasty of the trapezium, and arthrodesis. Various types of prosthetic joints have been developed for surgical replacement of the metacarpophalangeal and interphalangeal joints of the fingers including the CMC joint of the thumb.
There have been a number of total CMC joint implants wherein implants described and marketed having articulating surfaces have been surgically inserted into both the base of the first metacarpal and the trapezium. Other alternatives include replacing the entire trapezium, either along with the base of the metacarpus or by itself.
Many of the proposed artificial joints employ sliding between the prosthesis parts or between the prosthesis and the joint tissues. The sliding movement is typically accompanied by wear and abrasion of the joint parts, which in turn leads to inflammatory reactions, bone absorption, loosening of the prosthesis and dislocations.
An alternative approach for a prosthetic joint is suggested by U.S. Pat. No. 6,342,076, which employs a number of spring elements extending from end-to-end of the joint. The use of springs avoids the issues caused by sliding motion, and provides various advantages. Springs, however, when unlimited, may allow excessive motion in undesirable modes of motion, such as lateral shear, and may pose a risk of collapse if subjected to axial loading while laterally deformed.
Reviews of evidence-based researches concluded that there are no significant differences between all reported surgical solutions and the classic trapeziectomy when end results were compared. But, simple total trapeziectomy (the gold standard treatment) does not fully restore the CMC joint function. There is a proximal migration of the first metacarpal, resulting in a decrease in key pinch, tip pinch and grasp power. Pain is typically decreased but not eliminated.